There are 13 elements believed essential for plant growth (See Table below). They are divided into macronutrients and micronutrients depending on the relative optimum concentrations of each required by the plant. Generally, nutrient solutions used for growing terrestrial plants hydroponically are also suitable for growing aquatic plants, although in a much diluted form (generally 10%). The concentrations of various nutrients in a fertiliser will vary depending on the species being cultivated, but most weed species will grow well with high levels of nutrients added. The Table below lists concentration ranges of essential elements in hydroponic solutions. This list may be used as a base from which specialised recipes for particular species may be developed by experimentation. Very good recipes for nutrient solutions for hydroponic culture may also be found in Salisbury and Ross (1992) and Handreck and Black (1989).

Most submerged aquatic plants absorb nutrients in a water soluble form either;

Typically, submerged rosette-type plants (e.g. Sagittaria, Vallisneria, Blyxa) absorb most of their nutrients from the substrate, whereas elodeid-type plants (e.g. Elodea, Lagarosiphon, Myriophyllum, Egeria, Ceratophyllum) absorb a significant amount (although not all) of their mineral nutrient needs from the surrounding water (Barko et al. 1991). Therefore, for optimum growth, nutrients must be available to the plant in the water as well as in the substrate. Floating plants, with only their roots in water, do not require a fertilised substrate. They may be grown in simple plastic-lined pools filled with fertiliser added directly to the water. Conversely, emergent plants (e.g. Typha, Monochoria, reeds and sedges) and water lilies (Nymphaea spp.) derive all of their mineral nutrients from the substrate and require a well fertilised substrate.

Generally, the substrate is the primary source of phosphorus, iron, manganese and most micronutrients, while the open water is the primary source of calcium, magnesium, sodium, potassium, sulfate, and chloride (Barko et al. 1991), although varying levels of each can be found in both media. Nitrogen is found in a number of forms (nitrate, NO₃^-, and ammonium, NH₄⁺, the most common for plant uptake) in both the sediment and open water.

Increasing the levels of nutrients, in particular nitrogen and phosphorus, result in increased growth in most aquatic plants. There are many accounts in the literature of the effect of nutrient additions to the growth of nuisance aquatic plants, including: Eichhornia crassipes (Reddy et al. 1989; Reddy et al. 1990; Aoyama and Nishizaki 1993), Trapa natans (Tsuchiya and Iwaki 1983; Tsuchiya and Iwakuma 1993), Azolla spp. (Subudhi and Watanabe 1981; Cary and Weerts 1992), Elodea canadensis (Madsen and Baattrup-pedersen 1995), Salvinia molesta (Room 1985; Room and Thomas 1986; Forno and Semple 1987) and Myriophyllum spp. (Barko 1983; Chambers and Kalff 1985).

The composition of nutrient fertilisers for aquatic plants and their method of application to plants in an aquatic medium is quite different to those for terrestrial plants. The behaviour of nutrients in water and the chemical form in which they are found is very different to a soil medium. Water is generally an oxidising environment (i.e. oxygen rich) and elements such as iron (Fe), magnesium (Mg), phosphorus (P), etc. readily react with oxygen to form insoluble compounds (precipitates) which the plant cannot absorb. To keep these elements in solution longer, and thus available to the plant longer, compounds called chelates must be added. The most common form used is Ethylene-diamine-tetra acetic acid (EDTA). In the substrate, where oxygen tensions are low, iron is mostly in its soluble form and therefore plants rooted in the substrate rarely need treatments of iron fertiliser to the water column.

Problems may occur when there is a deficiency or overdose (toxicity) in one or more nutrients. Deficiencies in particular nutrients usually become manifest as characteristic external symptoms which may include any combination of:

A general diagnostic key to the identification of particular deficiency symptoms is presented in Salisbury and Ross (1992), with a more specific key to water hyacinth deficiency symptoms presented in Newman and Haller (1988).

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